Who's the King of the Dead?

yes as Finwe said hes the Leader of the army of the Dead, and whom Aragorn must convince to help them in the battle and make good their oath so they can be fulfilled (If I remember correctly...)

but a word to the wise: you really might want to read the entire book for yourself, you cant always trust what people tell you, and you really cant get the whole effect of the story if you just hear bits and pieces from other people...